procmail whitelist

I want to use procmail to whitelist. I have procmail up and running.  My intensions for this is if a sender is in the whitelist then they will not get scanned by spamassassin. As of right now I have spamassassin doing the whitelisting but spamassassin takes up to much resources. If spamassassin would first check to see if the sender is in a the whitelist and cancle the spamassassin scan if the sender is in the white list then my problmes would be solved.

I know that there are certian implications when doing this, such as spoofed senders but I am working on postfix to take care of this sort of thing. I have seen about 10 different recipes for procmail for whitelisting that can do this for me. I came accross a good discusion about this in the procmail mailing list, the Topic is Simplest Whitelist?.
http://www.xray.mpe.mpg.de/mailing-lists/procmail/2003-11/threads.html#00207
I understand all of these recipes except for this one
http://www.xray.mpe.mpg.de/mailing-lists/procmail/2003-11/msg00207.html
it seems to be a 1 liner
* ? test -x "$GREEN/$RP"
but I don't quite understand what is going on. Would someone be kind eoungh to explain what is going on.